In traditional ground-based cellular radio communication systems, base stations are stationary and subscriber units are mobile. Because base stations are stationary, shadowing is static. A subscriber unit in a given location is either unshadowed (able to communicate with at least one of the base stations) or shadowed (able to communicate with none of the base stations).
The problem of communicating with shadowed subscriber units in a system with static shadowing is readily solved by utilizing a traditional repeater. Such a repeater is located so as to be itself unshadowed while simultaneously unshadowing subscriber units that would otherwise be shadowed.
If the base stations are also mobile, e.g. in low-earth orbit satellite-based systems, shadowing is both static and dynamic. A subscriber unit in a given location may be unshadowed (able to communicate with at least one of the base stations at all times), dynamically shadowed (able to communicate with at least one of the base stations at some times and able to communicate with none of the base stations at other times), or statically shadowed (able to communicate with none of the base stations at all times).
A traditional repeater, being essentially a pair of transceivers coupled back to back, is for the most part unsuitable for use in systems with mobile base stations. A repeater location that is itself unshadowed while simultaneously accommodating the unshadowing of the subscriber units may not exist when dynamic shadowing occurs. When dynamic shadowing occurs, a conventional repeater arrangement would require cascaded repeaters to achieve unimpeded communication, leading to timing, interference, and fiscal problems. Similarly, when a traditional repeater is used with a dynamically shadowed subscriber unit, the subscriber unit experiences a conflict of signals between the repeater and a base station at least part of the time, with subsequent interference problems.
With a traditional repeater, there is no practical way for the system to determine which communications pass through the repeater and which do not. The cost of the repeater and its maintenance must then be applied to the system as a whole and disbursed among all subscribers. Hence, a subscriber never using a given repeater would nonetheless be paying for it.
What is needed is a repeater capable of functioning in a system with both mobile base stations and mobile subscriber units, hence with both dynamically and statically shadowed devices. What is also needed is a repeater which provides data so that the cost does not need to be amortized over all users.